Every day, you flush a liter or two of urine down the toilet. But if humans are going to get to Mars, they won’t be able to afford throwing out this yellow water. Indeed, they are going to have to drink water from their own pee. Scientists have now built a recycling system that can turn astronauts’ urine into both clean drinking water and energy.

That two-step process could be important in making long-distance space travel possible, report chemist Eduardo Nicolau of the University of Puerto Ri...